There are various kinds of commercially-available semiconductor memory devices. Memories with relatively larger capacity and lower power consumption include an SRAM (static random access memory). The SRAM has problems, however, because variations in threshold voltage result in unstable operation, and leakage current flowing in turned-off transistors increases power consumption.
On the other hand, semiconductor memory devices suitable for high integration include a DRAM. A method of forming a DRAM with multi-layered wiring for much higher integration is disclosed in Japanese Patent Laying-open No. 2002-133892, which is incorporated herein by reference. The DRAM thus formed usually requires formation of capacitors. Since the capacitors needs processes different from a CMOS processes. Accordingly, it is difficult to manufacture the DRAM through CMOS process only.
Therefore, it is advantageous in a semiconductor memory device that a DRAM can be formed through a general CMOS process, and to provide a DRAM-combined semiconductor memory device manufacturable at a lower production cost.